The invention relates to a method for optimizing the power input of a plurality of hysteresis motors connected in parallel.
In uranium enrichment plants, centrifuges are driven by hysteresis motors which are supplied with a medium-frequency three phase alternating voltage from, for example, a static frequency converter. In such arrangements the magnitude of the alternating voltage is regulated so that the maximum motor torque capable of being generated is greater, with a distinct safety margin, than the torque required by the method during synchronous running. The ratio of maximum torque to normal load torque is usually 1.5:1 when all motors are averaged, and 2:1 in the extreme case. This safety margin is necessary because many hundreds of centrifuge motors are supplied by one frequency converter, and synchronous operation must be ensured for all the centrifuges despite the variations occasioned during manufacture. Short term load increases may occur during operation of the plant because of malfunctions, during which the centrifuges are required to remain in synchronism. In addition, after a power supply failure, during which the centrifuges run down because of the lack of driving energy, an automatic return to synchronism is required in order to keep the interruption in operation as short as possible.
It is known from German Offenlegungsschrift (laid open application) No. 2 402 423, published Apr. 4th, 1976, and German Offenlegungsschrift No. 2 428 053, published May 20th, 1976, to monitor the synchronous running of each hysteresis motor in a plurality of parallel connected hysteresis motors, by means of a system which records the running thereof. Such systems use the phenomenon that the phase difference between motor current and motor voltage is dependent on motor load. The phase difference is used to determine the existence of a malfunctioning motor by detecting and evaluating the zero crossings of current and voltage for each individual motor and comparing the phase difference obtained therefor with a reference value which corresponds to the case of disturbed operation. For this purpose, the analog current signals coming from current transformers are initially converted into rectangular wave signals corresponding therewith in time, which signals are then supplied to a central evaluation logic via digital multiplexers. In the event of a malfunction, a signal giving the address of the malfunctioning motor is supplied to the central observation center.
This method requires that the output voltage of the power supply, which is produced by a static frequency converter, be kept constant within very narrow tolerances. If, however, in order to save energy, the output voltage of the frequency converter is to be adapted to the actual load condition of the motors, this method fails because identical phase relationships occur between current and voltage during both normal operation and during disturbed operation. Consequently the evaluation logic can no longer distinguish between these operating conditions.